Robust rank-two beamforming for multicell multigroup multicast

This paper investigates robust rank-two multicast beamforming for multiple groups of users in multicell systems. We consider the practical scenario when perfect channel state information is not available at the base stations. The channel uncertainty is assumed to be bounded within a spherical region. Robust beamforming is designed to guarantee that the predefined signal-to-interference-plus-noise ratio targets of all users are met for any channel instance in the bounded region. The Alamouti code is employed to double the degrees of freedom in the beamformer design. We apply the semidefinite relaxation (SDR) technique to address this nonconvex beamforming problem. Interestingly, our theoretical analysis shows that the SDR is tight when the number of users in each multicast group is no more than two and the channel uncertainty is sufficiently small.